1. Field of the Invention
This invention relates generally to an injection molding apparatus and more particularly to an independent-stroke ejection system for use in an injection molding apparatus.
2. Description of the Prior Art
In the formation of articles by an injection molding process, plasticized material is forced under considerable pressure into a closed mold wherein the material cools and hardens. As the moveable portion of the mold is retracted after hardening of the plastic material into a molded article, the molded article is typically ejected by means of a series of ejector pins. Rapid ejection of the molded article from the core during this knock-out portion of the injection molding cycle is desirable to avoid undue adhering of the article to the core as a result of shrinkage.
When the molded article is ejected, or knocked out, of the mold, in order to clear the mold for the manufacture of a subsequent molded article, any runners not part of the molded article must also be cleared from the mold. A runner, as such is understood by those skilled in the art, is produced by a channel leading, in the mold, from the nozzle or hot manifold for the molten plastic material to the actual portion of the mold used to manufacture the article.
Several different systems have been used over the years to knock molded articles and their associated runners from molds. One such system uses a delayed knock-out pin to eject the runner. This system includes a short, spring-loaded knock-out pin for the runner, which ejects the runner after the part. This system still performs very well, but suffers from one distinct disadvantage--the runner frequently becomes lodged within the molded article.
Another system uses a long knock-out pin for the runner. The long pin knocks the runner out first, before other shorter pins knock out the entire molded article. Although the system eliminates the problem of the runner being lodged within the molded article, several other problems are introduced. In particular, at the full knock-out forward position, the molded article may be ejected out of the mold cavity onto the long knock-out pin. The article may then hang on the end of the long knock-out pin after being ejected from the mold cavity. Sometimes, the long knock-out pin drags the molded article back into the mold cavity far enough to leave the part supported therein. This situation leaves open the possibility that the mold may close onto an already molded article. A second knock-out stroke is sometimes used to eliminate this possibility.
Prior-art systems such as these have also been used where molded articles of different sizes are being molded within the same family mold. In such cases, the several molded articles may each have different draw lengths; that is to say, they may require knock-out strokes of different length. The molding of articles of different sizes in a common mold is a widely used technique in the industry, but has associated with it problems similar to those discussed above in connection with the knocking-out of runners.
Similarly, these prior-art systems have also been used to knock out shallow portions of a molded product that has both shallow-draw and deep-draw areas. This has especially been the case where there is a possibility that the knock-out pins might interfere or make contact with core pulls or other common features of a mold.
The present invention has been designed to avoid the difficulties inherent in prior-art ejector systems, while maintaining their positive aspects. The present invention also provides an alternative means for knocking molded articles of various sizes from a family mold, and for knocking out molded articles having shallow-draw and deep-draw areas.